🤖 Agentic pull request

This was written by Claude (Opus 5), driven by @bjlittle. The commit carries a
Co-Authored-By trailer. Please review it as you would any other contribution.

Found while investigating the tests (py3.14) failure on #7276, which was
unrelated to that pull request.

The failure

tools/test_release_do_nothing.py::TestMergeBack::test_next_patch_file failed
on one job and no other. Re-running the identical commit passed. That pattern is
the diagnosis: it is a race between tests, not a bug in the code under test.

What races

IrisRelease.merge_back writes a progress file for the next patch. Unlike
every other nothing progress file, it is named after that patch instead of
being date-stamped — tools/release_do_nothing.py:1292:

next_patch_stem = self._get_file_stem().with_stem(next_patch_str)

nothing.Progress._get_file_stem resolves .nothing/ against Path().cwd(),
which every pytest-xdist worker shares. So the directory is the only thing
distinguishing one test's progress file from another's, and it does not vary.

Two tests reach that line with the same version, and so with the same path,
.nothing/v1_1_1.json:

• TestMergeBack::test_branches, in its more_patches parametrisation
• TestMergeBack::test_next_patch_file

Both set git_tag = "v1.0.1" and patch_min_max_tag = ("v1.0.1", "v1.2.1"),
which makes the next patch v1.1.1 in each case.

It is a race rather than a plain clash because the writes are not atomic and the
reads are not guarded:

• NextPatch(...) writes the JSON with write_text and then verifies it by
  loading it straight back.
• Its logger opens the sibling .log with mode="w", truncating it.
• test_next_patch_file loads the same path and asserts on its contents.

pyproject.toml sets no --dist, so xdist uses dynamic scheduling. Whether
those two tests land on the same worker varies from run to run, which is exactly
the intermittency observed.

The fix

A module-level autouse fixture giving each test its own working directory.

Autouse is the right scope rather than patching the two colliding tests:
_get_file_stem runs during __post_init__, so every construction of
IrisRelease is exposed to the shared directory, not just today's two
offenders.

Moving the working directory is safe here. All three subprocess calls in
release_do_nothing.py are the git helpers (_git_remote_v,
_git_remote_get_url, _git_ls_remote_tags), each already mocked by an autouse
fixture; every other path the module builds is anchored to Path(__file__).

It also stops the suite writing .nothing/ into the working copy as a side
effect — which is why **/.nothing is in .gitignore (.gitignore:85). On
main, pytest tools leaves .nothing/v1_1_1.json and v1_1_1.log in the
repository root. With this change it leaves nothing behind.

On testing a race

The failure is scheduling-dependent and cannot be reproduced on demand, so
test_progress_files_are_isolated asserts the isolation that prevents it rather
than the failure itself.

It is a real regression test: with the fixture body removed it fails, and names
the repository root in the message.

E  AssertionError: assert False
E   +  where False = is_relative_to(PosixPath('/tmp/pytest-of-.../test_progress_files_are_isolat0'))
E   +    where is_relative_to = PosixPath('/.../iris/.nothing/IrisRelease_20260911-105227').is_relative_to

Testing

pytest tools gives 122 passed, 1 skipped, against 121 passed, 1 skipped on
main — the one addition is the new test. Passes both serially and under
-n auto, and leaves no .nothing/ in the repository.

Note that pytest tools needs the nothing package, which is installed only by
noxfile.py and appears in no requirements or lock file. That is #7277, raised
separately for the core developers to rule on; it is an observation, not a
blocker for this.

Why this targets main

This is a repository-wide CI fix, not part of the merge/concatenate programme
that the greenfield feature branch carries.

greenfield needs it too — it inherits the same test module and the same
failure. It should pick it up by mergeback from main, not a cherry-pick,
so the branch keeps a single ancestry and does not carry a duplicate commit that
would have to be resolved later. greenfield descends from main, so the merge
is clean. @bjlittle and I will raise that mergeback once this lands.